Method of manufacturing an electro-optical device

ABSTRACT

An object of the invention is to reduce the manufacturing cost of EL display devices and electronic devices incorporating the EL display devices. An EL material is formed by printing in an active matrix EL display device. Relief printing or screen printing may be used as the method of printing. Manufacturing steps of the EL layer is therefore simplified and reduction of manufacturing cost is devised.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electro-optical device typified byan EL (electroluminescence) display device formed by fabricatingsemiconductor elements (elements using a semiconductor thin film,typically thin film transistors) on the surface of a substrate, and anelectronic device (electronic equipment) including the electro-opticaldevice as a display. Particularly, the invention relates to a method ofmanufacturing the same.

2. Description of the Related Art

In recent years, a technique for forming a thin film transistor(hereinafter referred to as a “TFT”) on a substrate has made remarkableprogress, and its application and development to an active matrix typedisplay device has proceeded. Especially, since a TFT using apolysilicon film has a field effect mobility higher than a conventionalTFT using an amorphous silicon film, a high speed operation can be made.Thus, it becomes possible to control a pixel, which has beenconventionally controlled by a driving circuit external to a substrate,by a driving circuit formed on the same substrate as the pixel.

Attention has been paid to such an active matrix type display devicesince various merits, such as reduction of manufacturing cost,miniaturization of a display device, increase of yield, and reduction ofthroughput, can be obtained by forming various circuits and elements onthe same substrate.

In the active matrix type EL display device, a switching element made ofa TFT is provided for each pixel, and a driving element for makingcurrent control is operated by the switching element, so that an ELlayer (light emitting layer) is made to emit light. For example, thereis an EL display device disclosed in U.S. Pat. No. 5,684,365 (seeJapanese Patent Application Laid-open No. Hei 8-234683), or JapanesePatent Application Laid-open Publication No. Hei 10-189252.

As a method of forming the EL layer, various methods have been proposed.For example, there can be enumerated vacuum evaporation, sputtering,spin coating, roll coating, a cast method, LB method, ion plating, adipping method, an inkjet method, and the like.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce a manufacturing cost ofan EL layer and to provide an inexpensive EL display device. Anotherobject of the invention is to reduce a product cost of an electronicdevice (electronic equipment) including the EL display device as adisplay.

In order to achieve the foregoing objects, the present invention ischaracterized in that an EL layer is formed by printing. A reliefprinting or a screen printing can be used as the printing method, and arelief printing is specifically preferable. The case of using a reliefprinting in the present invention is described here by using FIG. 1.

Shown in FIGS. 1A to 1C are a part of a relief printing apparatus usedin the present invention. In FIGS. 1A to 1C, reference numeral 110denotes an anilox roll; 111, a doctor bar (also referred to as a doctorblade); mixture of an EL element and a solvent (hereinafter referred toas EL forming substance) 112 is pooled at around the surface of theanilox roll 110 by the doctor bar 111. Note that the EL materialreferred here is a fluorescent organic compound and denotes an organiccompound which is referred to a hole injection layer, a hole transportlayer, a light emitting layer, an electron transport layer or anelectron injection layer in general.

Meshed grooves (hereinafter referred to as mesh) 110 a are provided onthe surface of the anilox roll 110 as shown in FIG. 1B, and the mesh 110a holds the EL forming substance 112 on its surface by rolling in adirection of arrow A. Note that the dotted line shown in the figuremeans that the EL forming substance is held on the surface of the aniloxroll 110.

Reference numeral 113 is a printing roll and 114, a relief, andunevenness is formed on the surface of the relief 114 by etching, etc.Such state is shown in FIG. 1C. In the case of FIG. 1C, patterns for thepixel section 114 a are formed in plural parts on the relief 114 inorder to manufacture a plurality of EL display devices over onesubstrate. Further, projections 114 b are formed at positionscorresponding to a plurality of pixels when a pattern for the pixelsection 114 a is enlarged.

The above stated anilox roll 110 keeps the EL forming substance 112 onthe mesh 110 a by rolling. On the other hand, the printing roll 113turns in a direction of an arrow B and only the projections 114 b of therelief 114 contact the mesh 110 a. Here, EL forming substance 112 iscoated on the surface of the projections 114 b.

EL forming substance 112 is printed at the sections where theprojections 114 b and a substrate 115 that is shifted in horizontaldirection (direction of an arrow C) at the same speed as the printingroll 113 contacts. By doing so, EL forming substance 112 is printed onthe substrate 115 in a state of arrangement into a matrix.

EL material is then resided by evaporating the solvent included in theEL forming substance 112 through heat treatment in a vacuum. It istherefore necessary to use a solvent that evaporates at a lowertemperature than the glass transition temperature (Tg) of the ELmaterial. The thickness of the finally formed EL layer is determined bythe viscosity of the EL forming substance. In this case the velocity canbe controlled by selection of the solvent, and the velocity of 10 to 50cp (preferably 20 to 30 cp) is preferable.

Further, the possibility of crystallizing the EL material throughevaporation of solvent becomes high when much impurities that can be acrystalline nucleus exist in the EL forming substance 112. Thecrystallization reduces the light emitting efficiency and thus it is notpreferable. It is preferable that least possible impurity is containedin the EL forming substance 112.

It is important to make the environment clean as possible, at refiningthe solvent, refining the EL material or mixing the solvent and the ELmaterial for reducing the impurities, and it is also preferable to payattention to the atmosphere on printing the EL forming substance by theprinting apparatus of FIG. 1. In concrete, it is preferable to performthe above stated printing process of the EL forming substance by aprinting apparatus placed in a clean booth filled with an inert gas suchas nitrogen.

Note that the present invention can be implemented to both of an activematrix EL display device and a passive matrix (simple matrix) EL displaydevice.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are diagrams for explaining the principle of a reliefprinting method;

FIG. 2 is a diagram showing a sectional structure of a pixel portion ofan EL display device;

FIGS. 3A and 3B are diagrams showing a top view of a pixel portion of anEL display device and a circuit structure thereof;

FIGS. 4A to 4E are diagrams showing manufacturing steps of an activematrix type EL display device;

FIGS. 5A to 5D are diagrams showing manufacturing steps of an activematrix type EL display device;

FIGS. 6A to 6C are diagrams showing manufacturing steps of an activematrix type EL display device;

FIG. 7 is a diagram showing the outer appearance of an EL module;

FIG. 8 is a diagram showing a circuit block structure of an EL displaydevice;

FIG. 9 is an enlarged view of a pixel portion of an EL display device;

FIG. 10 is a diagram showing an element structure of a sampling circuitof an EL display device;

FIGS. 11A and 11B are diagrams showing the outer appearance and a crosssectional view of an EL module;

FIGS. 12A to 12C are diagrams showing manufacturing steps of a contactstructure;

FIG. 13 is a diagram showing a structure at a pixel portion of an ELdisplay device;

FIG. 14 is a diagram showing a sectional structure of a pixel portion ofan EL display device;

FIG. 15 is a diagram showing a structure at a pixel portion of an ELdisplay device; and

FIGS. 16A to 16F are diagrams showing concrete examples of electronicdevices.

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode

A mode for carrying out the present invention will next be describedwith reference to FIG. 2 and FIGS. 3A and 3B. FIG. 2 is a sectional viewof a pixel portion of an EL display device of the present invention,FIG. 3A is a top view thereof, and FIG. 3B is a view showing its circuitstructure. A plurality of pixels are arranged in matrix form in practiceso that a pixel portion (image display portion) is formed. Note thatFIG. 2 corresponds to a sectional view taken along line A-A′ of FIG. 3A.Thus, since common symbols are used in FIG. 2 and FIG. 3A, both drawingsmay be suitably referred. Besides, although the top view of FIG. 3 showstwo pixels, both have the same structure.

In FIG. 2, a reference numeral 11 designates a substrate; and 12, aninsulating film (hereinafter referred to as a base film) which becomesan undercoat. A glass substrate, a glass ceramic substrate, a quartzsubstrate, a silicon substrate, a ceramic substrate, a metal substrate,or a plastic substrate (including a plastic film as well) can be used asthe substrate 11.

Although the base film 12 is effective especially in the case of usingthe substrate containing a movable ion or the substrate havingconductivity, it is not necessarily provided on the quartz substrate. Asthe base film 12, an insulating film containing silicon may be used.Note that through the specification, the “insulating film containingsilicon” indicates an insulating film containing silicon, oxygen andnitrogen at a predetermined ratio, for example, a silicon oxide film, asilicon nitride film, or a silicon oxynitride film (indicated bySiOxNy).

It is effective to dissipate heat generation of the thin film transistor(hereinafter referred to as TFT) by making the base film 12 have a heatradiating effect in order to prevent deterioration of a TFT ordeterioration of an EL element. For the purpose of making the film havethe heat radiating effect, any well-known materials can be used.

Here, two TFTs are formed in the pixel. A reference numeral 201designates a TFT (hereinafter referred to as a switching TFT)functioning as a switching element; and 202, a TFT (hereinafter referredto as a current controlling TFT) functioning as a current controllingelement for controlling the amount of current flowing to the EL element.Both are formed out of an n-channel TFT.

Since the field effect mobility of an n-channel TFT is larger than thefield effect mobility of a p-channel TFT, its operation speed is highand a large current can be easily made to flow. When the same amount ofcurrent is made to flow, the TFT size of the n-channel TFT can be madesmaller. Thus, it is preferable to use the n-channel TFT as the currentcontrolling TFT since an effective area of a display portion becomeswide.

The p-channel TFT has merits that hot carrier injection hardly becomes aproblem and an off current value is low, and there have been reported anexample in which it is used as a switching TFT and an example in whichit is used as a current controlling TFT. However, the present inventionis characterized also in that, by making such a structure that thepositions of LDD regions are made different, the problems of the hotcarrier injection and the off current value are solved even in then-channel TFT, and all the TFTs in the pixel are made the n-channelTFTs.

However, it is not necessary to limit the switching TFT and the currentcontrolling TFT to the n-channel TFT in the present invention, and it ispossible to use a p-channel TFT for both or either one of them.

The switching TFT 201 includes a source region 13, a drain region 14,LDD regions 15 a to 15 d, an active layer including a high concentrationimpurity region 16 and channel formation regions 17 a and 17 b, a gateinsulating film 18, gate electrodes 19 a and 19 b, a first interlayerinsulating film 20, a source wiring line 21, and a drain wiring line 22.

Besides, as shown in FIG. 3A, the gate electrodes 19 a and 19 b are of adouble gate structure in which they are electrically connected through agate wiring line 211 formed of another material (material havingresistance lower than the gate electrodes 19 a and 19 b). Of course, inaddition to the double gate structure, a so-called multi-gate structure(structure including an active layer having two or more channelformation regions connected in series), such as a triple gate structure,etc., may be adopted. The multi-gate structure is extremely effective inreducing the off current value, and in the present invention, theswitching TFT 201 of the pixel is made the multi-gate structure so thatthe switching element having a low off current value is realized.

The active layer is formed out of a semiconductor film containing acrystal structure. That is, a single crystal semiconductor film may beused or a polycrystalline semiconductor film or microcrystallinesemiconductor film may be used. The gate insulating film 18 may beformed out of an insulating film containing silicon. Besides, anyconductive films can be used for the gate electrode, source wiring line,or drain wiring line.

Further, in the switching TFT 201, the LDD regions 15 a to 15 d areprovided not to overlap with the gate electrodes 19 a and 19 b, with thegate insulating film 18 put between the LDD regions and the gateelectrodes. Such structure is very effective in reducing the off currentvalue.

Incidentally, it is more desirable to provide an offset region (regionwhich is made of a semiconductor layer having the same composition asthe channel formation region and to which a gate voltage is not applied)between the channel formation region and the LDD region in order toreduce the off current. In the case of multi-gate structure having twoor more gate electrodes, a high concentration impurity region providedbetween the channel formation regions is effective in reducing the offcurrent value.

As described above, by using the TFT of the multi-gate structure as theswitching element 201 of the pixel, it is possible to realize theswitching element having a sufficiently low off current value. Thus,even if a capacitor as shown in FIG. 2 of Japanese Patent ApplicationLaid-open No. Hei 10-189252 is not provided, the gate voltage of thecurrent controlling TFT can be held for a sufficient time (an intervalbetween a selection and a next selection).

That is, it becomes possible to remove a capacitor which hasconventionally been a factor to narrow an effective light emitting area,and it becomes possible to widen the effective light emitting area. Thismeans that the picture quality of the EL display device can be madebright.

Next, the current controlling TFT 202 comprises a source region 31, adrain region 32, an active layer including an LDD region 33 and achannel formation region 34, a gate insulating film 18, a gate electrode35, the first interlayer insulating film 20, a source wiring line 36,and a drain wiring line 37. Although the gate electrode 35 is of asingle gate structure, a multi-gate structure may be adopted.

As shown in FIG. 2, the drain of the switching TFT is connected to thegate of the current controlling TFT. Specifically, the gate electrode 35of the current controlling TFT 202 is electrically connected to thedrain region 14 of the switching TFT 201 through the drain wiring line(may be called a connection wiring line) 22. The source wiring line 36is connected to a current supply line 212.

Although the current controlling TFT 202 is an element for controllingthe amount of current injected to an EL element 203, in view ofdeterioration of the EL element, it is not preferable to supply a largeamount of current. Thus, in order to prevent an excessive current fromflowing to the current controlling TFT 202, it is preferable to designthe channel length (L) to be rather long. Desirably, it is designed sothat the current becomes 0.5 to 2 μA (preferably 1 to 1.5 μA) per pixel.

In view of the above, as shown in FIG. 9, when the channel length of theswitching TFT is L1 (L1=L1a+L1b), the channel width is W1, the channellength of the current controlling TFT is L2, and the channel width isW2, it is preferable that W1 is made between 0.1 and 5 μm (typically 0.5to 2 μm), and W2 is made between 0.5 and 10 μm (typically 2 to 5 μm).Besides, it is preferable that L1 is made 0.2 to 18 μm (typically 2 to15 μm), and L2 is made 1 to 50 μm (typically 10 to 30 μm). However, thepresent invention is not limited to the above numerical values.

By selecting the range of these numerical values, all standards can becovered, from an EL display device having the pixel number of VGA class(640×480) to that of high vision class (1920×1080 or 1280×1024).

Besides, it is appropriate that the length (width) of the LDD regionformed in the switching TFT 201 is made 0.5 to 3.5 μm, typically 2.0 to2.5 μm.

Besides, the EL display device shown in FIG. 2 is characterized also inthat the LDD region 33 is provided between the drain region 32 and thechannel formation region 34, and the LDD region 33 includes a regionoverlapping with and a region not overlapping with the gate electrode35, with the gate insulating film 18 put between them, in the currentcontrolling TFT 202.

The current controlling TFT 202 supplies current for causing the ELelement 204 to emit light, and controls the supply amount to enable grayscale display. Thus, it is necessary to take a countermeasure againstdeterioration due to the hot carrier injection so that deteriorationdoes not occur even if current is supplied. When black is displayed,although the current controlling TFT 202 is turned off, at that time, ifan off current is high, clear black display becomes impossible, and thelowering of contrast or the like is caused. Thus, it is necessary tosuppress the off current value as well.

With respect to the deterioration due to the hot carrier injection, itis known that the structure where the LDD region overlaps with the gateelectrode is very effective. However, if the whole of the LDD region ismade to overlap with the gate electrode, the off current value isincreased. Thus, the present applicant contrives a new structure thatthe LDD region not overlapping with the gate electrode is provided inseries, so that the problems of the hot carrier countermeasure and theoff current value countermeasure are solved at the same time.

At this time, it is appropriate that the length of the LDD regionoverlapping with the gate electrode is made 0.1 to 3 μm (preferably 0.3to 1.5 μm). If the length is too long, parasitic capacity becomes large,and if too short, the effect of preventing the hot carrier becomes weak.Besides, it is appropriate that the length of the LDD region notoverlapping with the gate electrode is made 1.0 to 3.5 μm (preferably1.5 to 2.0 μm). If the length is too long, it becomes impossible to makea sufficient current flow, and if too short, the effect of lowering theoff current value becomes weak.

In the above structure, parasitic capacity is formed in the region wherethe gate electrode and the LDD region overlap with each other. Thus, itis preferable not to provide such region between the source region 31and the channel formation region 34. In the current controlling TFT,since the direction of flow of carriers (here, electrons) is always thesame, it is sufficient if the LDD region is provided at only the side ofthe drain region.

However, when the driving voltage of the current controlling TFT 202(voltage applied between the source region and the drain region) becomes10 V or less, the hot carrier injection hardly becomes problematic, sothat it is also possible to omit the LDD region 33. In that case, theactive layer is made of the source region 31, the drain region 32, andthe channel formation region 34.

From the viewpoint of increasing the amount of current which can be madeto flow, it is also effective to increase the film thickness (preferably50 to 100 nm, more preferably 60 to 80 nm) of the active layer(especially the channel formation region) of the current controlling TFT202. On the contrary, in the case of the switching TFT 201, from theviewpoint of decreasing the off current value, it is also effective todecease the film thickness (preferably 20 to 50 nm, more preferably 25to 40 nm) of the active layer (especially the channel formation region).

Next, a reference numeral 41 designates a first passivation film, and itis appropriate that the thickness is made 10 nm to 1 μm (preferably 200to 500 nm). As the material, it is possible to use an insulating filmcontaining silicon (especially a silicon oxynitride film or siliconnitride film is preferable). This passivation film 41 has a function ofprotecting the formed TFT against alkali metal or moisture. In the ELlayer finally provided above the TFT, alkali metal such as sodium iscontained. That is, the first passivation film 41 functions also as aprotecting film which prevents the alkali metal (movable ion) fromentering the TFT side.

It is also effective to prevent thermal deterioration of the EL layer bycausing the first passivation film 41 to have a heat radiating effect.However, in the EL display device of the structure of FIG. 2, sincelight is radiated to the side of the substrate 11, it is necessary thatthe first passivation film 41 is translucent. In the case where organicmaterial is used for the EL layer, since deterioration is caused bycombination with oxygen, it is desirable not to use an insulating filmwhich easily emits oxygen.

As a translucent material which prevents penetration of alkali metal andhas a heat radiating effect, there can be cited an insulating filmcontaining at least one element selected from B (boron), C (carbon) andN (nitrogen), and at least one element selected from Al (aluminum), Si(silicon) and P (phosphorus). For example, it is possible to use anitride of aluminum typified by aluminum nitride (AlxNy), carbide ofsilicon typified by silicon carbide (SixCy), nitride of silicon typifiedby silicon nitride (SixNy), nitride of boron typified by boron nitride(BxNy), or phosphide of boron typified by boron phosphide (BxPy). Anoxide of aluminum typified by aluminum oxide (AlxOy) is excellent intransparency, and its thermal conductivity is 20 Wm⁻¹K, so that it canbe said one of preferable materials. These materials have not only theforegoing effects but also an effect to prevent penetration of moisture.Incidentally, in the foregoing translucent materials, x and y arearbitrary integers.

Note that it is also possible to combine the above compound with anotherelement. For example, it is also possible to use aluminum nitride oxideindicated by AlNxOy by adding nitrogen to the aluminum oxide. Thismaterial also has the effect to prevent penetration of moisture oralkali metal in addition to the heat radiating effect. Incidentally, inthe above aluminum nitride oxide, x and y are arbitrary integers.

Besides, it is possible to use materials disclosed in Japanese PatentApplication Laid-open No. Sho 62-90260. That is, it is also possible touse an insulating film containing Si, Al, N, O, or M (M is at least onekind of rare-earth element, preferably at least one element selectedfrom Ce (cerium), Yb (ytterbium), Sm (samarium), Er (erbium), Y(yttrium), La (lantern), Gd (gadolinium), Dy (dysprosium), and Nd(neodymium)). These materials also have the effect to preventpenetration of moisture or alkali metal in addition to the heatradiating effect.

Besides, it is also possible to use a carbon film containing at least adiamond thin film or an amorphous carbon film (especially a film havingcharacteristics close to diamond, called diamond-like carbon or thelike). These have very high thermal conductivity and are very effectiveas a heat radiating layer. However, since the film becomes brown and itstransmissivity is decreased when the thickness becomes large, it ispreferable to use the film having the least possible thickness(preferably 5 to 100 nm).

Incidentally, since the primary object of the first passivation film 41is to protect the TFT against the alkali metal or moisture, the filmmust not spoil the effect. Thus, although a thin film made of thematerial having the foregoing heat radiating effect can be used alone,it is effective to laminate the thin film and an insulating film(typically a silicon nitride film (SixNy) or silicon oxynitride film(SiOxNy)) which can prevent penetration of alkali metal or moisture.Incidentally, in the silicon nitride film or silicon oxynitride film, xand y are arbitrary integers.

A second interlayer insulating film (may be called a planarization film)44 is formed on the first passivation film 41 so as to cover therespective TFTs, so that step formed by TFTs are flattened. As thesecond interlayer insulating film 44, an organic resin film ispreferable, and it is appropriate that polyimide, polyamide, acrylic,BCB (benzocyclobutene), or the like is used. Of course, as long assufficient flattening can be made, an inorganic film may be used.

It is very important to flatten the step due to the TFTs by the secondinterlayer insulating film 44. Since an EL layer formed later is verythin, there is a case where poor light emission occurs by the existenceof the step. Thus, it is preferable to make flattening before a pixelelectrode is formed so that the EL layer can be formed on the flattestpossible surface.

A reference numeral 45 designates a second passivation film and has avery important function of blocking the alkali metal diffusing from theEL element. It is appropriate that the film thickness is made 5 nm to 1μm (typically 20 to 300 nm). An insulating film capable of blocking thepenetration of the alkali metal is used as the second passivation film45. As a material, the material used for the first passivation film 41can be used.

The second passivation film 45 functions also as a heat radiating layerwhich dissipates heat generated in the EL element and serves to preventheat from being stored in the EL element. In the case where the secondinterlayer insulating film 44 is an organic resin film, since it is weakto heat, the second passivation film prevents the heat generated in theEL element from having a bad influence on the second interlayerinsulating film 44.

As described above, although it is effective to flatten the TFTs withthe organic resin film when the EL display device is manufactured, therehas been no conventional structure in which consideration is paid to thedeterioration of the organic resin film caused by heat generated in theEL element. In the present invention, the problem is solved by providingthe second passivation film 45, which can be said one of the features.

The second passivation film 45 prevents the deterioration due to heatand also functions as a protecting film to prevent the alkali metal inthe EL layer from diffusing to the side of the TFT, and further, alsofunctions as a protecting layer to prevent moisture or oxygen frompenetrating from the side of the TFT to the side of the EL layer.

As stated above, separation of TFT side and EL element side by aninsulating film that has a high heat radiating effect and that iscapable of preventing penetration of moisture and alkali metals, is oneof the most important characteristics of the present invention, and itcan be said that such constitution did not exist in conventional ELdisplay devices.

A reference numeral 46 designates a pixel electrode (anode of the ELelement) made of a transparent conductive film. After a contact hole(opening) is formed through the second passivation film 45, the secondinterlayer insulating film 44, and the first passivation film 41, thepixel electrode is formed to be connected with the drain wiring line 37of the current controlling TFT 202 at the formed opening portion.

Next, an EL layer (strictly, an EL layer being in contact with the pixelelectrode) 47 is formed by printing. Although the EL layer 47 is used asa single layer structure or laminate structure, it is used as thelaminate structure in many cases. However, in the case of lamination, itis preferable to combine the printing and vapor phase growth(especially, an evaporation method is preferable). In the printingmethod, since a solvent and an EL material are mixed and are printed, ifan organic material is included underneath, there is a fear that theorganic material dissolves again.

Thus, it is preferable that a layer coming in direct contact with thepixel electrode within the EL layer 47 is formed by the printing, andlayer thereafter is formed by the vapor phase growth. Needless to say,if printing can be made by using such solvent that the EL material, ofthe lower layer does not dissolve, all layers can be formed by theprinting. A hole injecting layer, a hole transporting layer, or a lightemitting layer can be given as a layer coming in direct contact with thepixel electrode, the present invention can be used in a case of formingany layer.

In the present invention, since the printing is used as the method offorming the EL layer, it is preferable to use a polymer material as theEL material. As typical polymer materials, polymer materials such aspolyparaphenylene vinylene (PPV), polyvinylcarbazole (PVK) orpolyfluorene, can be enumerated.

In order to form the hole injecting layer, hole transporting layer, orlight emitting layer made of the polymer material by printing, printingis made in the state of a polymer precursor, and it is heated in avacuum and is converted into the EL material made of the polymermaterial. A necessary EL material is laminated thereon by theevaporation method or the like, so that the laminate type EL layer isformed.

Specifically, as the hole transporting layer, it is preferable thatpolytetrahydrothiophenylphenylene as the polymer precursor is used andis converted to polyphenylene vinylene by heating. It is appropriatethat the film thickness is made 30 to 100 nm (preferably 40 to 80 nm).As the light emitting layer, it is preferable that cyanopolyphenylenevinylene is used for a red light emitting layer, polyphenylene vinyleneis used for a green light emitting layer, and polyphenylene vinylene orpolyalkylphenylene is used for a blue light emitting layer. It isappropriate that the film thickness is made 0.30 to 150 nm (preferably40 to 100 nm).

It is also effective to provide copper phthalocyanine as a buffer layerbetween the pixel electrode and the EL material formed thereon.

However, the above examples are merely examples of organic EL materialswhich can be used as the EL materials of the present invention, and itis not necessary to limit the invention to these. In the presentinvention, a mixture of an EL material and a solvent is printed, and thesolvent is vaporized and removed, so that the EL layer is formed. Thus,as long as the combination is such that vaporization of the solvent iscarried out at a temperature not exceeding the glass transitiontemperature of the EL layer, any EL material may be used. Typically, asthe solvent, an organic solvent such as chloroform, dichloromethane,a-butyl lactone, butyl cellosolve, or NMP (N-methyl-2-pyrolidone) may beused, or water may be used. It is also effective to add an additive forincreasing the viscosity of the EL forming material.

Besides, when the EL layer 47 is formed, it is preferable that atreatment atmosphere is made a dry atmosphere with the least possiblemoisture and formation is carried out in an inert gas. Since the ELlayer is easily deteriorated by the existence of moisture or oxygen,when the layer is formed, it is necessary to remove such factors to theutmost. For example, a dry nitrogen atmosphere or dry argon atmosphereis preferable. For that purpose, it is desirable that the printingappratus is set in a clean booth filled with an inert gas, and theprinting treatment is carried out in the atmosphere.

When the EL layer 47 is formed by printing as described above, next, acathode 48 and a protecting electrode 49 are formed. The cathode 48 andthe protecting electrode 49 may be formed by a vacuum evaporationmethod. If the cathode 48 and the protecting electrode 49 arecontinuously formed without opening to the air, deterioration of the ELlayer can be further suppressed. In the present specification, a lightemitting element formed out of the pixel electrode (anode), the EL layerand the cathode is called the EL element.

As the cathode 48, a material containing magnesium (Mg), lithium (Li),or calcium (Ca) having a low work function is used. Preferably, anelectrode made of MgAg (material of Mg and Ag mixed at a ratio ofMg:Ag=10:1) is used. In addition, a MgAgAl electrode, a LiAl electrode,and a LiFAl electrode can be enumerated. The protective electrode 49 isan electrode, which is provided to protect the cathode 48 from outsidemoisture or the like, and a material containing aluminum (Al) or silver(Ag) is used. This protective electrode 49 has also a heat radiationeffect.

Incidentally, it is preferable that the EL layer 47 and the cathode 48are continuously formed in a dry inert gas atmosphere without opening tothe air. In the case where an organic material is used for the EL layer,since it is very weak to moisture, this way is adopted to avoid moistureabsorption at the time of opening to the air. Further, it is moredesirable to continuously form not only the EL layer 47 and the cathode48 but also the protective electrode 49 thereon.

The structure of FIG. 2 is an example of a case of using a monochromaticlight emitting system where one kind of EL element corresponding to anyone of RGB is formed. Although FIG. 2 shows only one pixel, a pluralityof pixels having the same structure are arranged in matrix form in thepixel portion. Incidentally, a well-known material may be adopted forthe EL layer corresponding to any one of RGB.

In addition to the above system, color display can be made by using asystem in which an EL element of white light emission and a color filterare combined, a system in which an EL element of blue or blue-greenlight emission and a fluorescent material (fluorescent color convertinglayer: CCM) are combined, a system in which a transparent electrode isused as a cathode (counter electrode) and EL elements corresponding toRGB are stacked, or the like. Of course, it is also possible to makeblack-and-white display by forming an EL layer of white light emissionin a single layer.

Reference numeral 50 designates a third passivation film, and it isappropriate that its film thickness is made 10 nm to 1 μm (preferably200 to 500 nm). Although a main object of providing the thirdpassivation film 50 is to protect the EL layer 47 from moisture, a heatradiation effect may also be provided, similarly to the secondpassivation film 45. Accordingly, a similar material to the firstpassivation film 41 can be used as a forming material. However, in thecase where an organic material is used for the EL layer 47, because thelayer may possibly be deteriorated through combination with oxygen, itis desirable not to use an insulating film, which is apt to give offoxygen.

Besides, as described above, since the EL layer is weak to heat, it isdesirable to form a film at the lowest possible temperature (preferablyin a temperature range of from room temperature to 120° C.). Thus, itcan be said that plasma CVD, sputtering, vacuum evaporation, ionplating, or a solution application method (spin coating method) is apreferable film forming method.

Like this, although the deterioration of the EL element can besufficiently suppressed by merely providing the second passivation film45, preferably, the EL element is surrounded by two-layer insulatingfilms formed to be put at both sides of the EL element, such as thesecond passivation film 45 and the third passivation film 50, so thatintrusion of moisture and oxygen into the EL layer is prevented,diffusion of alkaline metal from the EL layer is prevented, and storageof heat into the EL layer is prevented. As a result, deterioration ofthe EL layer is further suppressed, and an EL display device having highreliability can be obtained.

The EL display device of the present invention includes a pixel portionmade of a pixel having a structure as in FIG. 2, and TFTs havingdifferent structures according to functions are disposed in the pixel.Accordingly, a switching TFT having a sufficiently low off current valueand a current controlling TFT strong against hot carrier injection canbe formed within the same pixel, and an EL display device having highreliability and enabling excellent picture display (having highoperation performance) can be obtained.

Note that in the pixel structure of FIG. 2, although a TFT having amulti-gate structure is used as the switching TFT, it is not necessaryto limit a structure of arrangement of LDD regions or the like to thestructure of FIG. 2.

Further, though an example of implementing the present invention informing an EL element that is electrically connected to a TFT providedas a semiconductor device on a substrate surface is shown here, it isalso possible to implement the present invention in the case of usingtransistors formed on a silicon substrate surface (referred to asMOSFET) as semiconductor devices.

The present invention manufactured through the foregoing constituentswill be described in more detail with reference to embodiments describedbelow.

Embodiment 1

The embodiments of the present invention are explained using FIGS. 4A to6C. A method of simultaneous manufacture of a pixel portion, and TFTs ofa driver circuit portion formed in the periphery of the pixel portion,is explained here. Note that in order to simplify the explanation, aCMOS circuit is shown as a basic circuit for the driver circuits.

First, as shown in FIG. 4A, a base film 301 is formed with a 300 nmthickness on a glass substrate 300. Silicon oxynitride films arelaminated as the base film 301 in embodiment 1. It is good to set thenitrogen concentration at between 10 and 25 wt % in the film contactingthe glass substrate 300.

Besides, as a part of the base film 301, it is effective to provide aninsulating film made of a material similar to the first passivation film41 shown in FIG. 2. The current controlling TFT is apt to generate heatsince a large current is made to flow, and it is effective to provide aninsulating film having a heat radiating effect at a place as close aspossible.

Next, an amorphous silicon film (not shown in the figures) is formedwith a thickness of 50 nm on the base film 301 by a known depositionmethod. Note that it is not necessary to limit this to the amorphoussilicon film, and another film may be formed provided that it is asemiconductor film containing an amorphous structure (including amicrocrystalline semiconductor film). In addition, a compoundsemiconductor film containing an amorphous structure, such as anamorphous silicon germanium film, may also be used. Further, the filmthickness may be made from 20 to 100 nm.

The amorphous silicon film is then crystallized by a known method,forming a crystalline silicon film (also referred to as apolycrystalline silicon film or a polysilicon film) 302. Thermalcrystallization using an electric furnace, laser annealingcrystallization using a laser, and lamp annealing crystallization usingan infrared lamp exist as known crystallization methods. Crystallizationis performed in embodiment 1 using light from an excimer laser whichuses XeCl gas.

Note that pulse emission type excimer laser light formed into a linearshape is used in embodiment 1, but a rectangular shape may also be used,and continuous emission argon laser light and continuous emissionexcimer laser light can also be used.

In this embodiment, although the crystalline silicon film is used as theactive layer of the TFT, it is also possible to use an amorphous siliconfilm. However, in order to increase an aperture ratio of a pixel bymaking an area of the current controlling TFT as small as possible, itis advantageous to use the crystalline silicon film through which acurrent can easily flow.

Note that it is effective to form the active layer of the switching TFT,in which there is a necessity to reduce the off current, by theamorphous silicon film, and to form the active layer of the currentcontrol TFT by the crystalline silicon film. Electric current flows withdifficulty in the amorphous silicon film because the carrier mobility islow, and the off current does not easily flow. In other words, the mostcan be made of the advantages of both the amorphous silicon film,through which current does not flow easily, and the crystalline siliconfilm, through which current easily flows.

Next, as shown in FIG. 4B, a protecting film 303 is formed on thecrystalline silicon film 302 with a silicon oxide film having athickness of 130 nm. This thickness may be chosen within the range of100 to 200 nm (preferably between 130 and 170 nm). Furthermore, otherfilms may also be used provided that they are insulating filmscontaining silicon. The protecting film 303 is formed so that thecrystalline silicon film is not directly exposed to plasma duringaddition of an impurity, and so that it is possible to have delicateconcentration control of the impurity.

Resist masks 304 a and 304 b are then formed on the protecting film 303,and an impurity element which imparts n-type conductivity (hereafterreferred to as an n-type impurity element) is added through theprotecting film 303. Note that elements residing in periodic table group15 are generally used as the n-type impurity element, and typicallyphosphorous or arsenic can be used. Note that a plasma doping method isused, in which phosphine (PH₃) is plasma activated without separation ofmass, and phosphorous is added at a concentration of 1×10¹⁸ atoms/cm³ inembodiment 1. An ion implantation method, in which separation of mass isperformed, may also be used, of course.

The dose amount is regulated so that the n-type impurity element iscontained in n-type impurity regions 305 and 306, thus formed by thisprocess, at a concentration of 2×10¹⁶ to 5×10¹⁹ atoms/cm³ (typicallybetween 5×10¹⁷ and 5×10¹⁸ atoms/cm³).

Next, as shown in FIG. 4C, the protecting film 303 is removed, and anactivation of the added periodic table group 15 elements is performed. Aknown technique of activation may be used as the means of activation,and activation is performed in embodiment 1 by irradiation of excimerlaser light. A pulse emission type excimer laser is and a continuousemission type excimer laser may both, of course, be used, and it is notnecessary to place any limits on the use of excimer laser light. Thegoal is the activation of the added impurity element, and it ispreferable that irradiation is performed at an energy level at which thecrystalline silicon film does not melt. Note that the laser irradiationmay also be performed with the protecting film 303 in place. Theactivation by heat treatment may also be performed along with activationof the impurity element by laser light. When activation is performed byheat treatment, considering the heat resistance of the substrate, it isgood to perform heat treatment on the order of 450 to 550° C.

A boundary portion (connecting portion) with regions along the edges ofthe n-type impurity regions 305 and 306, namely regions along theperimeter into which the n-type impurity element, which exists in then-type impurity regions 305 and 306, is not added, is defined by thisprocess. This means that, at the point when the TFTs are latercompleted, extremely good connections can be formed between LDD regionsand channel forming regions.

Unnecessary portions of the crystalline silicon film are removed next,as shown in FIG. 4D, and island shape semiconductor films (hereafterreferred to as active layers) 307 to 310 are formed.

Then, as shown in FIG. 4E, a gate insulating film 311 is formed,covering the active layers 307 to 310. An insulating film containingsilicon and with a thickness of 10 to 200 nm, preferably between 50 and150 nm, may be used as the gate insulating film 311. A single layerstructure or a lamination structure may be used. A 110 nm thick siliconoxynitride film is used in embodiment 1.

A conducting film with a thickness of 200 to 400 nm is formed next andpatterned, forming gate electrodes 312 to 316. Note that in embodiment1, the gate electrodes and lead wirings electrically connected to thegate electrodes (hereafter referred to as gate wirings) are formed fromdifferent materials. Specifically, a material having a lower resistancethan that of the gate electrodes is used for the gate wirings. This isbecause a material which is capable of being micro-processed is used asthe gate electrodes, and even if the gate wirings cannot bemicro-processed, the material used for the wirings has low resistance.Of course, the gate electrodes and the gate wirings may also be formedfrom the same material.

Further, the gate wirings may be formed by a single layer conductingfilm, and when necessary, it is preferable to use a two layer or a threelayer lamination film. All known conducting films can be used as thegate electrode material. However, as stated above, it is preferable touse a material which is capable of being micro-processed, specifically,a material which is capable of being patterned to a line width of 2 mmor less.

Typically, it is possible to use a film comprising an element selectedfrom tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W),chromium (Cr), and silicon (Si), a film of nitride of the above element(typically a tantalum nitride film, tungsten nitride film, or titaniumnitride film), an alloy film of combination of the above elements(typically Mo—W alloy, Mo—Ta alloy), or a silicide film of the aboveelement (typically a tungsten silicide film, titanium silicide film). Ofcourse, the films may be used as a single layer or a laminate layer.

In this embodiment, a laminate film of a tungsten nitride (WN) filmhaving a thickness of 50 nm and a tungsten (W) film having a thicknessof 350 nm is used. These may be formed by sputtering. When an inert gasof Xe, Ne or the like is added as a sputtering gas, film peeling due tostress can be prevented.

The gate electrodes 313 and 316 are formed at this time so as to overlapa portion of the n-type impurity regions 305 and 306, respectively,sandwiching the gate insulating film 311. This overlapping portion laterbecomes an LDD region overlapping the gate electrode.

Next, an n-type impurity element (phosphorous is used in embodiment 1)is added in a self-aligning manner with the gate electrodes 312 to 316as masks, as shown in FIG. 5A. The addition is regulated so thatphosphorous is added to impurity regions 317 to 323 thus formed at aconcentration of 1/10 to ½ that of the n-type impurity regions 305 and306 (typically between ¼ and ⅓). Specifically, a concentration of 1×10¹⁶to 5×10¹⁸ atoms/cm³ (typically 3×10¹⁷ to 3×10¹⁸ atoms/cm³) ispreferable.

Resist masks 324 a to 324 d are formed next, with a shape covering thegate electrodes etc, as shown in FIG. 5B, and an n-type impurity element(phosphorous is used in embodiment 1) is added, forming impurity regions325 to 331 containing a high concentration of phosphorous. Ion dopingusing phosphine (PH₃) is also performed here, and is regulated so thatthe phosphorous concentration of these regions is from 1×10²⁰ to 1×10²¹atoms/cm³ (typically between 2×10²⁰ and 5×10²⁰ atoms/cm³).

A source region or a drain region of the n-channel TFT is formed by thisprocess, and in the switching TFT, a portion of the n-type impurityregions 320 to 322 formed by the process of FIG. 5A remains. Theseremaining regions correspond to the LDD regions 15 a to 15 d of theswitching TFT in FIG. 2.

Next, as shown in FIG. 5C, the resist masks 324 a to 324 d are removed,and a new resist mask 332 is formed. A p-type impurity element (boron isused in embodiment 1) is then added, forming impurity regions 333 and334 containing a high concentration of boron. Boron is added here toform impurity regions 333 and 334 at a concentration of 3×10²⁰ to 3×10²¹atoms/cm³ (typically between 5×10²⁰ and 1×10²¹ atoms/cm³) by ion dopingusing diborane (B₂H₆).

Note that phosphorous has already been added to the impurity regions 333and 334 at a concentration of 1×10¹⁶ to 5×10¹⁸ atoms/cm³, but boron isadded here at a concentration of at least 3 times more than that of thephosphorous. Therefore, the n-type impurity regions already formedcompletely invert to p-type, and function as p-type impurity regions.

Next, after removing the resist mask 332, the n-type and p-type impurityelements added at various concentrations are activated. Furnaceannealing, laser annealing, or lamp annealing may be performed as ameans of activation. Heat treatment is performed in embodiment 1 in anitrogen atmosphere for 4 hours at 550° C. in an electric furnace.

It is important to remove as much of the oxygen in the atmosphere aspossible at this time. This is because if any oxygen exists, then theexposed surface of the gate electrode oxidizes, inviting an increase inresistance, and at the same time it becomes more difficult to later makean ohmic contact. It is therefore preferable that the concentration ofoxygen in the processing environment in the above activation processshould be 1 ppm or less, preferably 0.1 ppm or less.

After the activation process is completed, a gate wiring 335 with athickness of 300 nm is formed next. A metallic film having aluminum (Al)or copper (Cu) as its principal constituent (comprising 50 to 100% ofthe composition) may be used as the material of the gate wiring 335. Aswith the gate wiring 211 of FIG. 3, the gate wiring 335 is formed with aplacement so that the gate electrodes 314 and 315 of the switching TFTs(corresponding to gate electrodes 19 a and 19 b of FIG. 3) areelectrically connected. (See FIG. 5D.)

The wiring resistance of the gate wiring can be made extremely small byusing this type of structure, and therefore a pixel display region(pixel portion) having a large surface area can be formed. Namely, thepixel structure of embodiment 1 is extremely effective because an ELdisplay device having a screen size of a 10 inch diagonal or larger (inaddition, a 30 inch or larger diagonal) is realized due to thisstructure.

A first interlayer insulating film 336 is formed next, as shown in FIG.6A. A single layer insulating film containing silicon is used as thefirst interlayer insulating film 336, while a lamination film may becombined in between. Further, a film thickness of between 400 nm and 1.5μm may be used. A lamination structure of an 800 nm thick silicon oxidefilm on a 200 nm thick silicon oxynitride film is used in embodiment 1.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an environment containing between 3 and 100% hydrogen,performing hydrogenation. This process is one of hydrogen termination ofdangling bonds in the semiconductor film by hydrogen which is thermallyactivated. Plasma hydrogenation (using hydrogen activated by a plasma)may also be performed as another means of hydrogenation.

Note that the hydrogenation step may also be inserted during theformation of the first interlayer insulating film 336. Namely, hydrogenprocessing may be performed as above after forming the 200 nm thicksilicon oxynitride film, and then the remaining 800 nm thick siliconoxide film may be formed.

Next, a contact hole is formed in the first interlayer insulating film336, and source wiring lines 337 to 340 and drain wiring lines 341 to343 are formed. In this embodiment, this electrode is made of a laminatefilm of three-layer structure in which a titanium film having athickness of 100 nm, an aluminum film containing titanium and having athickness of 300 nm, and a titanium film having a thickness of 150 nmare continuously formed by sputtering. Of course, other conductive filmsmay be used.

A first passivation film 344 is formed next with a thickness of 50 to500 nm (typically between 200 and 300 nm). A 300 nm thick siliconoxynitride film is used as the first passivation film 344 inembodiment 1. This may also be substituted by a silicon nitride film. Itis of course possible to use the same materials as those of the firstpassivation film 41 of FIG. 2.

Note that it is effective to perform plasma processing using a gascontaining hydrogen such as H₂ or NH₃ etc. before the formation of thesilicon oxynitride film. Hydrogen activated by this preprocess issupplied to the first interlayer insulating film 336, and the filmquality of the first passivation film 344 is improved by performing heattreatment. At the same time, the hydrogen added to the first interlayerinsulating film 336 diffuses to the lower side, and the active layerscan be hydrogenated effectively.

Next, as shown in FIG. 6B, a second interlayer insulating film 347 madeof organic resin is formed. As the organic resin, it is possible to usepolyimide, polyamide, acrylic, BCB (benzocyclobutene) or the like.Especially, since the second interlayer insulating film 347 is primarilyused for flattening, acrylic, which is excellent in flatteningproperties is preferable. In this embodiment, an acrylic film is formedto a thickness sufficient to flatten a stepped portion formed by TFTs.It is appropriate that the thickness is preferably made 1 to 5 μm (morepreferably, 2 to 4 μm).

Next, a second passivation film 348 having a thickness of 100 nm isformed on the second interlayer insulating film 347. In this embodiment,since an insulating film containing Si, Al, N, O and La is used, it ispossible to prevent alkaline metal from diffusing from the EL layerprovided thereon. At the same time, intrusion of moisture into the ELlayer is blocked and heat generated in the EL layer is dissipated, sothat it is possible to suppress deterioration of the EL layer due toheat and deterioration of the flattened film (second interlayerinsulating film).

A contact hole reaching a drain wiring line 343 is formed through thesecond passivation film 348, the second interlayer insulating film 347,and the first passivation film 344, and a pixel electrode 349 is formed.In this embodiment, an indium-tin oxide (ITO) film having a thickness of110 nm is formed, and patterning is carried out to form the pixelelectrode. This pixel electrode 349 becomes an anode of the EL element.Incidentally, as other materials, it is also possible to use anindium-titanium oxide film or a film of ITO mixed with zinc oxide.

Incidentally, this embodiment has such a structure that the pixelelectrode 349 is electrically connected to the drain region 331 of thecurrent controlling TFT through the drain wiring line 343. Thisstructure has merits as set forth below.

Since the pixel electrode 349 comes in direct contact with an organicmaterial of an EL layer (light emitting layer) or charge transportinglayer, there is a possibility that a movable ion contained in the ELlayer or the like diffuses in the pixel electrode. That is, in thestructure of this embodiment, the pixel electrode 349 is not directlyconnected to the drain region 331 as a part of the active layer, but thedrain wiring line 343 is made to intervene so that intrusion of themovable ion into the active layer can be prevented.

Next, as shown in FIG. 6C, an EL layer 350 is formed by printingexplained by use of FIG. 1, and further, a cathode (MgAg electrode) 351and a protecting electrode 352 are formed by evaporation without openingto the air. At this time, it is preferable to completely remove moistureby carrying out a heat treatment to the pixel electrode 349 before theEL layer 350 and the cathode 351 are formed. In this embodiment,although the MgAg electrode is used as the cathode of the EL element,other well-known materials are also acceptable.

As the EL layer 350, the materials explained in the Embodiment Modesection can be used. In this embodiment, although a two-layer structureof a hole transporting layer and a light emitting layer is made an ELlayer, there is also a case where either one of a hole injecting layer,an electron injecting layer, or an electron transporting layer isprovided. Like this, various examples have already been reported for thecombination, and any structure of them may be used.

In this embodiment, as the hole transporting layer,polytetrahydrothiophenylphenylene that is a polymer precursor, is formedby a printing method and is converted to polyphenylene vinylene byheating. As the light emitting layer, a material obtained by moleculardispersion of PBD of 1,3,4-oxadiazole derivatives of 30 to 40% intopolyvinylcarbazole is formed by evaporation, and coumarin 6 of about 1%is added as the center of green light emission.

Although even the protective electrode 352 can protect the EL layer 350from moisture or oxygen, preferably, a third passivation film 353 may beprovided. In this embodiment, as the third passivation film 353, asilicon nitride film having a thickness of 300 nm is provided. Thisthird passivation film may also be formed continuously after theprotective electrode 352 without opening to the air. Of course, as thethird passivation film 353, the same material as the third passivationfilm 50 of FIG. 2 may be used.

Besides, the protective electrode 352 is provided to prevent degradationof the MgAg electrode 351, and a metal film containing aluminum as itsmain ingredient is typical. Of course, another material may be used.Since the EL layer 350 and the MgAg electrode 351 are very weak tomoisture, it is preferable to make continuous formation to theprotective electrode 352 without opening to the air so that the EL layeris protected from the outside air.

Incidentally, it is appropriate that the film thickness of the EL layer350 is made 10 to 400 nm (typically 60 to 150 nm, preferably 100 to 120nm), and the thickness of the MgAg electrode 351 is made 80 to 200 nm(typically 100 to 150 nm).

In this way, an active matrix EL display device having a structure asshown in FIG. 6C is completed. In the active matrix EL display device ofthis embodiment, TFTs having optimum structure are disposed in not onlythe pixel portion but also the driving circuit portion, so that veryhigh reliability is obtained and operation characteristics can also beimproved.

First, a TFT having a structure to decrease hot carrier injection so asnot to drop the operation speed thereof as much as possible is used asan n-channel TFT 205 of a CMOS circuit forming a driving circuit.Incidentally, the driving circuit here includes a shift register, abuffer, a level shifter, a sampling circuit (sample and hold circuit)and the like. In the case where digital driving is performed, a signalconversion circuit such as a D/A converter can also be included.

In the case of this embodiment, as shown in FIG. 6C, the active layer ofthe n-channel TFT 205 includes a source region 355, a drain region 356,an LDD region 357 and a channel formation region 358, and the LDD region357 overlaps with the gate electrode 313, putting the gate insulatingfilm 311 therebetween.

Consideration not to drop the operation speed is the reason why the LDDregion is formed at only the drain region side. In this n-channel TFT205, it is not necessary to pay attention to an off current value verymuch, rather, it is better to give importance to an operation speed.Thus, it is desirable that the LDD region 357 is made to completelyoverlap with the gate electrode to decrease a resistance component to aminimum. That is, it is preferable to remove the so-called offset.

In the p-channel TFT 206 of the CMOS circuit, since deterioration due tohot carrier injection can be almost neglected, an LDD region does nothave to be particularly provided. Of course, similarly to the n-channelTFT 205, it is also possible to provide an LDD region to take acountermeasure against hot carriers.

Incidentally, a sampling circuit among driving circuits is ratherspecific as compared with other circuits, and a large current flowsthrough a channel formation region in both directions. That is, theroles of a source region and a drain region are counterchanged. Further,it is necessary to suppress an off current value to the lowest possiblevalue, and in that meaning, it is preferable to dispose a TFT having anapproximately intermediate function between the switching TFT and thecurrent controlling TFT.

Thus, as an n-channel TFT forming the sampling circuit, it is preferableto dispose a TFT having a structure as shown in FIG. 10. As shown inFIG. 10, parts of LDD regions 901 a and 901 b overlap with a gateelectrode 903, through a gate insulating film 902. This effect is as setforth in the explanation of the current controlling TFT 202, and adifferent point is that in the sampling circuit, the LDD regions 901 aand 901 b are provided to be put at both sides of a channel formationregion 904.

When the state of FIG. 6C is completed, it is preferable in practice tomake packaging (sealing) by a housing member such as a protection filmhaving high airtightness and less degassing (laminate film, ultravioletray curing resin film, etc.) or a ceramic sealing can so as to preventexposure to the outer air. At that time, when the inside of the housingmember is made an inert gas atmosphere, or a moisture absorbent (forexample, barium oxide) is disposed in the inside, the reliability(lifetime) of the EL layer is improved.

After the airtightness is raised by processing such as packaging, aconnector (flexible print circuit: FPC) for connecting a terminalextended from the element or circuit formed on the substrate to anexternal signal terminal is attached so that a product is completed. Inthe present specification, the EL display device, which is made to havesuch a state that it can be shipped, is called an EL module.

Here, the structure of the active matrix EL display device of thisembodiment will be described with reference to a perspective view ofFIG. 7. The active matrix EL display device of this embodiment isconstituted by a pixel portion 602, a gate side driving circuit 603, anda source side driving circuit 604 formed over a glass substrate 601. Aswitching TFT 605 of a pixel portion is an n-channel TFT, and isdisposed at an intersection point of a gate wiring line 606 connected tothe gate side driving circuit 603 and a source wiring line 607 connectedto the source side driving circuit 604. The drain of the switching TFT605 is connected to the gate of a current controlling TFT 608.

Further, the source of the current controlling TFT 608 is connected to acurrent supply line 609, and an EL element 610 is connected to the drainof the current controlling TFT 608.

Input-output wiring lines (connection wiring lines) 612 and 613 fortransmitting signals to the driving circuits and an input-output wiringline 614 connected to the current supply line 609 are provided in an FPC611 as an external input-output terminal.

An example of circuit structure of the EL display device shown in FIG. 7is shown in FIG. 8. The EL display device of this embodiment includes asource side driving circuit 701, a gate side driving circuit (A) 707, agate side driving circuit (B) 711, and a pixel portion 706. Note thatthrough the specification, the term “driving circuit” is a generic termincluding the source side driving circuit and the gate side drivingcircuit.

The source side driving circuit 701 comprises a shift register 702, alevel shifter 703, a buffer 704, and a sampling circuit (sample and holdcircuit) 705. The gate side driving circuit (A) 707 comprises a shiftregister 708, a level shifter 709, and a buffer 710. The gate sidedriving circuit (B) 711 also has the similar structure.

Here, the shift registers 702 and 708 have driving voltages of 5 to 16 V(typically 10 V) respectively, and the structure indicated by 205 inFIG. 6C is suitable for an n-channel TFT used in a CMOS circuit formingthe circuit.

Besides, for each of the level shifters 703 and 709 and the buffers 704and 710, similarly to the shift register, the CMOS circuit including then-channel TFT 205 of FIG. 6C is suitable. Note that it is effective tomake a gate wiring line a multi-gate structure such as a double gatestructure or a triple gate structure in improvement of reliability ofeach circuit.

Further, since the source region and drain region are inverted and it isnecessary to decrease an off current value, a CMOS circuit including then-channel TFT 208 of FIG. 10 is suitable for the sampling circuit 705.

The pixel portion 706 are disposed pixels having the structure shown inFIG. 2.

The foregoing structure can be easily realized by manufacturing TFTs inaccordance with the manufacturing steps shown in FIGS. 4A to 6C. In thisembodiment, although only the structure of the pixel portion and thedriving circuit is shown, if the manufacturing steps of this embodimentare used, it is possible to form a logic circuit other than the drivingcircuit, such as a signal dividing circuit, a D/A converter circuit, anoperational amplifier circuit, a ã-correction circuit, or the like onthe same substrate, and further, it is presumed that a memory portion, amicroprocessor, or the like can be formed.

Further, an EL module of this embodiment including a housing member aswell will be described with reference to FIGS. 11A and 11B.Incidentally, as needed, reference numbers used in FIGS. 7 and 8 will bequoted.

A pixel portion 1701, a source side driving circuit 1702, and a gateside driving circuit 1703 are formed on a substrate (including a basefilm below a TFT) 1700. Each wiring line from the respective drivingcircuits lead to an FPC 611 through input wiring lines 612 to 614 andare connected to an external equipment.

At this time, a housing member 1704 is provided to surround at least thepixel portion, preferably the driving circuit and the pixel portion. Thehousing member 1704 has such a shape as to have a recess portion with aninner size (depth) larger than an outer size (height) of the pixelportion 1701 or a sheet shape, and is fixed by an adhesive 1705 to thesubstrate 1700 so as to form an airtight space in cooperation with thesubstrate 1700. At this time, the EL element is put in such a state thatit is completely sealed in said airtight space, and is completely shutoff from the outer air. Incidentally, a plurality of housing members1704 may be provided.

As a material of the housing member 1704, an insulating material such asglass or polymer is preferable. For example, amorphous glass(boro-silicate glass, quartz, etc.), crystallized glass, ceramic glass,organic resin (acrylic resin, styrene resin, polycarbonate resin, epoxyresin, etc.), and silicone resin are enumerated. Besides, ceramics maybe used. If the adhesive 1705 is an insulating material, a metalmaterial such as a stainless alloy can also be used.

As a material of the adhesive 1705, an adhesive of epoxy resin, acrylateresin, or the like can be used. Further, thermosetting resin orphoto-curing resin can also be used as the adhesive. However, it isnecessary to use such material as to block penetration of oxygen andmoisture to the utmost.

Further, it is preferable that a space 1706 between the housing memberand the substrate 1700 is filled with an inert gas (argon, helium,nitrogen, etc.). Other than the gas, an inert liquid (liquid fluorinatedcarbon typified by perfluoroalkane, etc.) can also be used. With respectto the inert liquid, a material as used in Japanese Patent ApplicationLaid-open No. Hei 8-78159 may be used.

It is also effective to provide a drying agent in the space 1706. As thedrying agent, a material as disclosed in Japanese Patent ApplicationLaid-open No. Hei 9-148066 can be used. Typically, barium oxide may beused.

Besides, as shown in FIG. 11B, a plurality of pixels each includingisolated EL elements is provided in a pixel portion, and all of theminclude a protective electrode 1707 as a common electrode. In thisembodiment, although the description has been made such that it ispreferable to continuously form the EL layer, the cathode (MgAgelectrode) and the protective electrode without opening to the air, ifthe EL layer and the cathode are formed by using the same mask member,and only the protective electrode is formed by a different mask member,the structure of FIG. 11B can be realized.

At this time, the EL layer and the cathode may be formed only on thepixel portion, and it is not necessary to provide them on the drivingcircuit. Of course, although a problem does not arise if they areprovided on the driving circuit, when it is taken into considerationthat alkaline metal is contained in the EL layer, it is preferable notto provide.

Incidentally, the protective electrode 1707 is connected to an inputwiring line 1709 in a region indicated by 1708. The input wiring line1709 is a wiring line to give a predetermined voltage (in thisembodiment, earthing potential, concretely 0V) to the protectiveelectrode 1707, and is connected to the FPC 611 through a conductivepaste material 1710.

Here, manufacturing steps for realizing a contact structure in theregion 1708 will be described with reference to FIG. 12.

First, in accordance with the steps of this embodiment, the state ofFIG. 6A is obtained. At this time, at an end portion of the substrate(region indicated by 1708 in FIG. 11B), the first interlayer insulatingfilm 336 and the gate insulating film 311 are removed, and an inputwiring line 1709 is formed thereon. Of course, it is formed at the sametime as the source wiring line and the drain wiring line of FIG. 6A(FIG. 12A).

Next, in FIG. 6B, when the second passivation film 348, the secondinterlayer insulating film 347, and the first passivation film 344 areetched, a region indicated by 1801 is removed, and an opening portion1802 is formed (FIG. 12B).

In this state, in the pixel portion, a forming step of an EL element(forming step of a pixel electrode, an EL layer and a cathode) iscarried out. At this time, in the region shown in FIG. 12, a mask memberis used so that the EL element is not formed. After a cathode 351 isformed, a protective electrode 352 is formed by using another maskmember. By this, the protective electrode 352 and the input wiring line1709 are electrically connected. Further, a third passivation film 353is provided to obtain the state of FIG. 12C.

Through the foregoing steps, the contact structure of the regionindicated by 1708 of FIG. 11B is realized. The input-output wiring line1709 is connected to the FPC 611 through a gap between the housingmember 1704 and the substrate 1700 (however, the gap is filled with theadhesive 1705. That is, the adhesive 1705 is required to have such athickness as to be able to sufficiently flatten unevenness due to theinput-output wiring line). Incidentally, although the description hasbeen made here on the input-output wiring line 1709, other output wiringlines 612 to 614 are also connected to the FPC 611 through the portionunder the housing member 1704 in the same manner.

Embodiment 2

In this embodiment, an example in which a structure of a pixel is madedifferent from the structure shown in FIG. 3B will be described withreference to FIG. 13.

In this embodiment, two pixels shown in FIG. 3B are arranged to becomesymmetrical with respect to a current supply line 212 for applyingground voltage. That is, as shown in FIG. 13, a current supply line 212is made common to two adjacent pixels, so that the number of necessarywiring lines can be reduced. Incidentally, a TFT structure or the likearranged in the pixel may remain the same.

If such structure is adopted, it becomes possible to manufacture a moreminute pixel portion, and the quality of an image is improved.

Note that the structure of this embodiment can be easily realized inaccordance with the manufacturing steps of the embodiment 1, and withrespect to the TFT structure or the like, the description of theembodiment 1 or FIG. 2 may be referred to.

Embodiment 3

In this embodiment, a case where a pixel portion having a structuredifferent from FIG. 2 will be described with reference to FIG. 14. Notethat the steps up to a step of forming a second interlayer insulatingfilm 44 may be carried out in accordance with the embodiment 1. Since aswitching TFT 201 and a current controlling TFT 202 covered with thesecond interlayer insulating film 44 have the same structure as that inFIG. 1, the description here is omitted.

In the case of this embodiment, after a contact hole is formed throughthe second passivation film 45, the second interlayer insulating film44, and the first passivation film 41, a pixel electrode 51 is formed,and then, a cathode 52 and an EL layer 53 are formed. In thisembodiment, after the cathode 52 is formed by vacuum evaporation, the ELlayer 53 is formed by relief printing or screen printing while a dryinert gas atmosphere is maintained.

In this embodiment, an aluminum alloy film (aluminum film containingtitanium of 1 wt %) having a thickness of 150 nm is provided as thepixel electrode 51. As a material of the pixel electrode, although anymaterial may be used as long as it is a metal material, it is preferablethat the material has high reflectivity. A MgAg electrode having athickness of 120 nm is used as the cathode 52, and the thickness of theEL layer 53 is made 120 nm.

In this embodiment, an EL forming substance is manufactured by mixing asolvent to EL material, that is obtained from molecular dispersion ofPBD of 1,3,4-oxadiazole derivatives of 30 to 40% into polyvinylcarbazoleand by adding coumarin 6 of about 1% as the center of light emission.The EL forming substance is applied by relief printing or screenprinting, and baking treatment is carried out, so that a green lightemitting layer having a thickness of 50 nm is obtained. TPD having athickness of 70 nm is formed thereon by evaporation and the EL layer 53is obtained.

Next, an anode 54 made of a transparent conductive film (in thisembodiment, an ITO film) is formed to a thickness of 110 nm. In thisway, an EL element 209 is formed, and when a third passivation film 55is formed by a material shown in the embodiment 1, the pixel having thestructure as shown in FIG. 14 is completed.

In the case where the structure of this embodiment is adopted, greenlight generated in each pixel is radiated to a side opposite to thesubstrate on which the TFT is formed. Thus, almost all regions in thepixel, that is, even the region where the TFT is formed can also be usedas an effective light emitting region. As a result, an effective lightemitting area of the pixel is greatly improved, and the brightness andcontrast ratio (ratio of light to shade) of an image is increased.

Incidentally, the structure of this embodiment can be freely combinedwith any of the embodiments 1 and 2.

Embodiment 4

Although the description has been made on the case of the top gate typeTFT in the embodiments 1 to 4, the present invention is not limited tothe TFT structure, and may be applied to a bottom gate type TFT(typically, reverse stagger type TFT). Besides, the reverse stagger typeTFT may be formed by any means.

Since the reverse stagger type TFT has such a structure that the numberof steps can be easily made smaller than the top gate type TFT, it isvery advantageous in reducing the manufacturing cost, which is theobject of the present invention. Incidentally, the structure of thisembodiment can be freely combined with any structure of the embodiments2 and 3.

Embodiment 5

It is effective to use a material having a high thermal radiatingeffect, similar to that of the second passivation film 45, as the basefilm formed between the active layer and the substrate in the structuresof FIG. 6C of embodiment 1 or FIG. 2. In particular, current flows inthe current control TFT for a long time, and therefore heat is easilygenerated, and deterioration due to self generation of heat can become aproblem. Thermal deterioration of the TFT can be prevented by using thebase film of embodiment 5, which has a thermal radiating effect, forthis type of case.

The effect of protecting from the diffusion of movable ions from thesubstrate is also very important, of course, and therefore it ispreferable to use a lamination structure of a compound including Si, Al,N, O, and M, and an insulating film containing silicon, similar to thefirst passivation film 41.

Note that it is possible to freely combine the constitution ofembodiment 5 with the constitution of any of embodiments 1 to 4.

Embodiment 6

When the pixel structure shown in embodiment 3 is used, the lightemitted from the EL layer is radiated in the direction opposite to thesubstrate, and therefore it is not necessary to pay attention to thetransmissivity of materials, such as the insulating film, which existbetween the substrate and the pixel electrode. In other words, materialswhich have a somewhat low transmissivity can also be used.

It is therefore advantageous to use a carbon film, such as one referredto as a diamond thin film or an amorphous carbon film, as the base film12, the first passivation film 41 or the second passivation film 45. Inother words, because it is not necessary to worry about lowering thetransmissivity, the film thickness can be set thick, to between 100 and500 nm, and it is possible to have a very high thermal radiating effect.

Regarding the use of the above carbon films in the third passivationfilm 50, note that a reduction in the transmissivity must be avoided,and therefore it is preferable to set the film thickness to between 5and 100 nm.

Note that, in embodiment 6, it is effective to laminate with anotherinsulating film when a carbon film is used in any of the base film 12,the first passivation film 41, the second passivation film 45 and thethird passivation film 50.

In addition, embodiment 6 is specifically effective when the pixelstructure shown in embodiment 3 is used, but it is also possible tofreely combine the constitution of embodiment 6 with the constitution ofany of embodiments 1, 2, 4 and 5.

Embodiment 7

The amount of the off current value in the switching TFT in the pixel ofthe EL display device is reduced by using a multi-gate structure for theswitching TFT, and the present invention is characterized by theelimination of the need for a storage capacitor. This is a device formaking good use of the surface area, reserved for the storage capacitor,as an emitting region.

However, even if the storage capacitor is not completely eliminated, aneffect of increasing the effective emitting surface area, by the amountthat the exclusive surface area is made smaller, can be obtained. Inother words, the object of the present invention can be sufficientlyachieved by reducing the value of the off current by using a multi-gatestructure for the switching TFT, and by only shrinking the exclusivesurface area of the storage capacitor.

In that case as shown in FIG. 15 it is acceptable to form a storagecapacitor 1401 in parallel to the gate of the current control TFT 202with respect to the drain of the switching TFT 201.

Note that the constitution of embodiment 7 can be freely combined withany constitution of embodiments 1 to 6. Namely, a storage capacitor ismerely formed within a pixel and it is not to limit the TFT structure,materials of EL layer, etc.

Embodiment 8

Laser crystallization is used as the means of forming the crystallinesilicon film 302 in embodiment 1, and a case of using a different meansof crystallization is explained in embodiment 8.

After forming an amorphous silicon film in embodiment 8, crystallizationis performed using the technique recorded in Japanese Patent ApplicationLaid-open No. Hei 7-130652. The technique recorded in the above patentapplication is one of obtaining a crystalline silicon film having goodcrystallinity by using an element such as nickel as a catalyst forpromoting crystallization.

Further, after the crystallization process is completed, a process ofremoving the catalyst used in the crystallization may be performed. Inthis case, the catalyst may be gettered using the technique recorded inJapanese Patent Application Laid-open No. Hei 10-270363 or JapanesePatent Application Laid-open No. Hei 8-330602.

In addition, a TFT may be formed using the technique recorded in thespecification of Japanese Patent Application No. Hei 11-076967 by theapplicant of the present invention.

The processes of manufacturing shown in embodiment 1 are one embodimentof the present invention, and provided that the structure of FIG. 2 orof FIG. 6C of embodiment 1 can be realized, then other manufacturingprocess may also be used without any problems, as above.

Note that it is possible to freely combine the constitution ofembodiment 8 with the constitution of any of embodiments 1 to 7.

Embodiment 9

In driving the EL display device of the present invention, analogdriving can be performed using an analog signal as an image signal, anddigital driving can be performed using a digital signal.

When analog driving is performed, the analog signal is sent to a sourcewiring of a switching TFT, and the analog signal, which contains grayscale information, becomes the gate voltage of a current control TFT.The current flowing in an EL element is then controlled by the currentcontrol TFT, the EL element light emitting intensity is controlled, andgray scale display is performed. Note that the current control TFT maybe operated in a saturation region in case of performing analog driving.

On the other hand, when digital driving is performed, it differs fromthe analog type gray scale display, and gray scale display is performedby time ratio gray scale method. Namely, by regulating the length of thelight emission time, color gray scales can be made to be seen visuallyas changing. In case of performing digital driving, it is preferable tooperate the current control TFT in the linear region.

The EL element has an extremely fast response speed in comparison to aliquid crystal element, and therefore it is possible to have high speeddriving. Therefore, the EL element is one which is suitable for timeratio gray scale method, in which one frame is partitioned into a pluralnumber of subframes and then gray scale display is performed.

The present invention is a technique related to the element structure,and therefore any method of driving it may thus be used.

Embodiment 10

In embodiment 1 it is preferable to use an organic EL material as an ELlayer, but the present invention can also be implemented using aninorganic EL material. However, current inorganic EL materials have anextremely high driving voltage, and therefore a TFT which has voltageresistance characteristics that can withstand the driving voltage mustbe used in cases of performing analog driving.

Alternatively, if inorganic EL materials having lower driving voltagesthan conventional inorganic EL materials are developed, then it ispossible to apply them to the present invention.

Further, it is possible to freely combine the constitution of embodiment10 with the constitution of any of embodiments 1 to 9.

Embodiment 11

An active matrix EL display device (EL module) formed by implementingthe present invention has superior visibility in bright locations incomparison to a liquid crystal display device because it is aself-emitting type device. It therefore has a wide range of uses as adirect-view type EL display (indicating a display incorporating an ELmodule).

Note that a wide viewing angle can be given as one advantage which theEL display has over a liquid crystal display. The EL display of thepresent invention may therefore be used as a display (display monitor)having a diagonal equal to 30 inches or greater (typically equal to 40inches or greater) for appreciation of TV broadcasts by large screen.

Further, not only can it be used as an EL display (such as a personalcomputer monitor, a TV broadcast reception monitor, or an advertisementdisplay monitor), it can be used as a display for various electronicdevices.

The following can be given as examples of such electronic devices: avideo camera; a digital camera; a goggle type display (head mounteddisplay); a car navigation system; a personal computer; a portableinformation terminal (such as a mobile computer, a mobile telephone, oran electronic book); and an image playback device using a recordingmedium (specifically, a device which performs playback of a recordingmedium and is provided with a display which can display those images,such as a compact disk (CD), a laser disk (LD), or a digital versatiledisk (DVD)). Examples of these electronic devices are shown in FIGS. 16Ato 16F.

FIG. 16A is a personal computer, comprising a main body 2001, a casing2002, a display portion 2003, and a keyboard 2004. The present inventioncan be used in the display portion 2003.

FIG. 16B is a video camera, comprising a main body 2101, a displayportion 2102, an audio input portion 2103, operation switches 2104, abattery 2105, and an image receiving portion 2106. The present inventioncan be used in the display portion 2102.

FIG. 16C is a portion of a head-mounted type EL display (right handside), comprising a main body 2201, a signal cable 2202, a fasting band2203, a display monitor 2204, an optical system 2205 and a displaydevice 2206. The present invention can be used in the display device2206.

FIG. 16D is an image playback device (specifically, a DVD playbackdevice) provided with a recording medium, comprising a main body 2301, arecording medium (such as a CD, an LD, or a DVD) 2302, operationswitches 2303, a display portion (a) 2304, and a display portion (b)2305. The display portion (a) is mainly used for displaying imageinformation, and the image portion (b) is mainly used for displayingcharacter information, and the present invention can be used in theimage portion (a) and in the image portion (b). Note that the presentinvention can be used as an image playback device provided with arecording medium in devices such as a CD playback device and gameequipment.

FIG. 16E is a mobile computer, comprising a main body 2401, a cameraportion 2402, an image receiving portion 2403, operation switches 2404,and a display portion 2405. The present invention can be used in thedisplay portion 2405.

FIG. 16F is an EL display, comprising a casing 2501, a support stand2502, and a display portion 2503. The present invention can be used inthe display portion 2503. Because EL displays have large viewing angle,they are especially advantageous for cases in which the screen is madelarge, and is favorable for displays having a diagonal greater than orequal to 10 inches (especially one which is greater than or equal to 30inches).

Furthermore, if the emission luminance of EL materials becomes higher infuture, then it will become possible to use the present invention in afront type or a rear type projector by enlarging and projecting thelight which includes outputted image information by a lens, etc.

The range of applications of the present invention is thus extremelywide, and it is possible to apply the present invention to electronicdevices in all fields. Furthermore, the electronic devices of embodiment11 can be realized by using any constitution of any combination ofembodiments 1 to 10.

By implementing the present invention, the formation of the EL layer canbe made at an extremely low cost. Thus, the manufacturing cost of the ELdisplay device can be reduced.

Besides, by providing the insulating film capable of preventingpenetration of alkali metal between the EL layer and the TFT, it ispossible to prevent the alkali metal from diffusing out of the EL layerand from badly influencing the TFT characteristics. As a result, theoperation performance and reliability of the EL display device can begreatly improved.

Besides, by using the EL display device, which can be manufactured at alow cost, as a display, the manufacturing cost of an electronic deviceis reduced. Besides, by using the EL display device in which theoperation performance and reliability are improved, it becomes possibleto produce an applied product (electronic device) having excellentpicture quality and durability (high reliability).

1. A method for manufacturing an active matrix display devicecomprising: forming a plurality of switching elements over a substrate;forming an insulating film over the plurality of switching elements;forming a plurality of pixel electrodes over the insulating film; andprinting an electroluminescence forming substance over the plurality ofpixel electrodes.